The increase in use of high quality audio equipment in automobiles and the like has generated a need for high-performance speaker systems which provide a substantially level audio signal for the full sound spectrum discernible by the human ear.
It is known that single cone speaker systems do not reproduce the minimum high-fidelity frequency range of 40 to 15,000 Hz with adequate smoothness of response, freedom from distortion, or power handling capacity. Because low frequency reproduction dictates the use of large, heavy cones, while high frequency reproduction requires the use of small, light-weight cones, it is possible to combine two such cones in one speaker drive mechanism, so that the low and middle frequencies will be reproduced by the large cone and the high frequencies will be radiated from the smaller cone. The two cone speakers are superior to the single cone system.
Although two cone systems represent a significant improvement in performance, they are still prone to special types of distortion known as intermodulation, or "Doppler" distortion. Because the middle and high frequencies (where the ear is most sensitive to distortion) are still being reproduced by the same drive mechanism which generates the low frequencies, the low frequencies tend to modulate or affect the frequency of the middle and high frequencies.
The low frequency modulation problem is solved by splitting the audio frequency spectrum into two parts and then reproducing each half with a speaker element designed just for that portion of the high-fidelity spectrum. It is known to mount two electrically independent speakers of this type on a common frame with the speaker elements coaxial. The coaxial speaker systems provide improved sound reproduction when compared to the dual cone design; however, since the middle range of frequencies is still mainly reproduced by the low frequency speaker, Doppler distortion and lack of suitable mid-range clarity still results.
It is known to further split the audio frequency spectrum into three parts with the low-range speaker element producing only the lowest end of the sound spectrum, the high-range speaker element specifically adapted for reproducing only the high end of the spectrum and a mid-range speaker for reproducing only the mid-range portion of the frequency spectrum, thereby substantially minimizing the Doppler distortion while increasing the mid-range clarity.
Early three-element speaker systems having three electrically independent driver and speaker elements were adapted for home and professional use and were large systems mounted on the three independent frames within a single enclosure. This was later replaced by a three-element speaker system wherein the three independent electrical driver and speaker elements are mounted on a common frame. These units are also intended for home entertainment and professional applications, and are quite large and heavy. Yet, the improved quality of sound reproduction produced by the three element speaker systems make them highly desirable despite their overall size and weight.
The evolution of compact speaker systems for use in automobiles and the like has recently very closely paralleled that of home high-fidelity speaker systems of years ago. Basic features such as specially molded speaker cones, large magnetic driver structures, high compliance suspension systems and high temperature voice coils, which are known to offer improved performance in home high-fidelity speakers have now been successfully introduced into compact speaker design. In addition, coaxial two-element speaker systems having electrically independent low-range and high-range elements mounted on a single frame are now available in compact systems for use in automobiles and the like.
It would seem logical that a three or more element compact speaker system for use in automobiles would provide improved quality of sound reproduction over the coaxial speaker for automobile systems just as it did in home and professional entertainment systems. However, the designing of such multiple-element speaker systems for automobiles presented a host of technological problems not soluble by known expedients. The speakers have to reproduce the frequency spectrum normally found in home or professional stereo speaker systems with the same quality and at the same time remain compact and be designed to fit within those spaces provided by automobile manufacturers. These problems are overcome by the present invention.